The U.S. Department of Defense is currently moving toward the long-term goal of insensitive munition-compliant inventory. The acquisition treatment of insensitive munitions was the subject of a Jan. 26, 1999, memorandum from the Under Secretary of Defense for Acquisition, Technology and Logistics. The overall intent of the memorandum was to focus scarce resources on forward fit incorporation of insensitive munition-compliant technology.
Insensitive munition is expected to save lives and materials. As defined in STANAG 4439, insensitive munitions mean: “Munitions which reliably fulfill their performance, readiness and operational requirements on demand, but which minimize the probability of inadvertent initiation and severity of subsequent collateral damage to weapon platforms, logistics systems and personnel when subjected to unplanned stimuli.”
“Unplanned stimuli” include thermal and mechanical impact threats of Fast Cook-Off (FCO), Slow Cook-Off (SCO), Bullet Impact (BI), Fragment Impact (FI), Sympathetic Detonation (SD), Shaped Charge Jet (SCJ), and Spall impact (SI) as presented in MIL-STD-2105B.
The memorandum adds: “All munitions and weapons shall be designed to conform with insensitive munitions (unplanned stimuli) criteria and to use materials consistent with safety and interoperability requirements. Requirements shall be determined during the requirements validation process and shall be kept current throughout the acquisition cycle for all acquisition programs. Interoperability, to include insensitive munitions policies, shall be certified per CJCSI 3170.01 A.” “The ultimate objective is to design and field munitions which have no adverse reaction to unplanned stimuli, analogous to Hazard Division 1.6 (TB 700-2/NAVSEAINST 8020.8B/T.O. 11A-1-47/DLAR 8220.1, “Department of Defense Ammunition and Explosives Hazard Classification Procedures”).”
While prior efforts to develop an insensitive munition (IM) propulsion system for a recoilless weapon utilizing a countermass presented certain advantages, they still suffered from numerous shortcomings, amongst which are the following:                a. Recoilless weapons often utilize filament wound barrels in order to maximize strength and minimize weight. Because cutting holes in these barrels would compromise their integrity, the common practice of venting the propulsion system will not be feasible.        b. The utilization of heat sensitive materials to allow the countermass to drain will be difficult because the countermass behaves as a heat sink, preventing the heat sensitive materials from heating durng a cook off.        c. Other features located within the barrel relied on heat to activate, but the percussion cap located on the surface of the barrel is exposed to the heat much earlier than the in-barrel features.        d. Some of these features utilized eutectic alloys to solder mechanical components. While eutectic alloys have an excellent temperature response, the processes to solder these components lack an industrial base for full rate production. Additionally, the eutectic alloy is costly.        
According to United States Code, Title 10, Chapter 141, Section 2389—Ensuring safety regarding Insensitive Munitions (IM)—the Secretary of Defense shall ensure, to the extent practicable, that munitions under development of procurement are safe throughout development and fielding when subject to unplanned stimuli.
Two tests are used to simulate munitions exposed to a fire: Slow Cook Off (SCO) and Fast Cook Off (FCC). In SCO, munitions in packaged configuration are heated at a rate of 6° C./hour until it reacts. In FCC, munitions are engulfed in a flame of at least 1700° C. until it reacts. It is desirable for the reaction to be limited to no more than burning (Type 5 reaction). A detonation is not acceptable (Type 1 reaction).
Recoilless weapons operate by using expanding propellant gases to propel a projectile forward and a mass backwards in order to minimize recoil. Some recoilless weapons utilize a fluid as a countermass, which is propelled backwards in order to minimize the hazard of the back blast to allow for firing from enclosure.
When tested for IM, the propulsion systems of the recoilless weapons sometimes ignite, launching the projectile, which fails IM requirements with a Type 4 deflagration. The warhead may become armed and detonate, which fails IM with a Type 1 detonation. It has been found that the removal of the countermass will often prevent the projectile from leaving the barrel and arming.
It would therefore be desirable to provide a bleeding mechanism for use in the propulsion system of a recoilless, insensitive munition (IM) utilizing a utilizing a fluidic countermass that addresses the foregoing problems associated with convention IM systems. The bleeding mechanism would be activated by excess heat, and as a result, it would cause the countermass container to rupture. Once ruptured, the countermass fluid will drain out. Without a countermass, the propulsion system will no longer function. The need for such a bleeding mechanism has heretofore remained unsatisfied.